1. Field of the Invention
The present invention relates to a drive control unit for a hydraulic elevator which makes a cage ascend and descend by a hydraulic pump at a variable speed and particularly to a drive control unit which prevents negative pressure from the hydraulic pump from occurring at the time of a descending operation.
2. Description of the Related Art
In a conventional hydraulic elevator in which a cage is made to ascend and descend by using a hydraulic jack, a speed of the cage is controlled in such a manner that an electric motor is driven at a fixed rotating speed at the time of an ascending operation and an amount of oil provided to the hydraulic jack, from a fixed amount of oil sucked from an oil tank and emitted from the hydraulic pump, is adjusted by a flow controlling valve. At the time of a descending operation, the speed of the cage is controlled in such a manner that an amount of oil, which has been forced back from the hydraulic jack to the oil tank, is adjusted by the flow controlling valve.
However, in such a control method, since an excess amount of oil which is not provided to the hydraulic jack drives back from the hydraulic pump to the oil tank at the time of the ascending operation, a great loss of energy occurs. At the time of descending operation, since potential energy is converted to heat, this method is ineffective and the oil temperature rises greatly.
It has been proposed to control variably the amount of oil emitted from the hydraulic pump in such a manner that an induction motor is controlled at a variable voltage and a variable frequency (hereinafter called VVVF) by using an inverter or the like and the hydraulic pump is driven by the induction motor, as shown in Japanese Patent Publication No. 64-311.
In accordance with the controlling method using such an inverter, since a required amount of oil corresponding to a speed command value is provided to the hydraulic jack during ascent and the induction motor is regeneratively driven by the amount of oil which has forced back to the oil tank during descent so that energy consumption is reduced, the rising oil temperature is controlled, and thus an efficient drive control unit for a hydraulic elevator can be provided.
FIG. 2 shows a construction of a conventional drive control unit for a hydraulic elevator in which an amount of oil emitted from the hydraulic pump is variably controlled by the VVVF control.
Referring now to FIG. 2, a cylinder 1 is provided in a pit of a hoistway of the elevator, pressure oil 2 is filled in the cylinder 1, and a plunger 3 is supported by the pressure oil 2 and is driven up and down. The hydraulic jack consists of the above members.
In FIG. 2, a deflector sheave 4 is rotatively provided at the top of the plunger 3; the end of a cable 5 is secured to the pit and the center portion thereof passes around the deflector sheave 4; and a cage 6, fastened to the other end of the cable 5, is driven up and down.
A solenoid-operated valve 7 is provided with an oil pipe connected to the cylinder 1 which closes when the cage 6 stops as shown in FIG. 2. When the elevator starts, the valve 7 is opened by energization of an electromagnetic coil to clear a passage to the hydraulic jack.
A hydraulic pump 9 is variably driven so that it sends and receives the pressure oil to and from the solenoid-operated valve 7. An oil tank 12 sends and receives the pressure oil to and from the hydraulic pump 9. A three-phase induction motor 14 drives the hydraulic pump 9. An inverter 15 drives the induction motor 14 by the VVVF control. Connected to the inverter 15 is a three-phase AC power source 16. A converter 17 converts AC voltage from the three-phase AC power source 16 to DC voltage. A smoothing capacitor 18 smooths DC voltage output from the converter 17 and applies it to the inverter 15. A regeneration resistor 19 consumes power regenerated from the inverter 15. A regeneration transistor 20 is energized when the inverter 15 is regenerated.
A speed detector 21 detects the rotating speed S of the induction motor 14. A pattern generator 22 outputs a speed command value Q corresponding to ascending and descending drive patterns. A speed control unit 23 transmits a torque command value T to the induction motor 14 in accordance with the deviation between the speed command value Q and the rotating speed S. A current detector 24 detects a primary current I flowing to the induction motor 14. A torque control unit 25 controls the inverter 21 according to the torque command value T, the rotating speed S and the primary current I.
An operation of a conventional drive control unit of a hydraulic elevator is shown in FIG. 2.
During ascent of the cage 6, the positive-polarity speed command value Q is input into the speed control unit 23 which generates the torque command value T according to the deviation between the speed command value Q and the actual rotating speed S of the induction motor 14.
The torque control unit 25 controls the inverter 15 according to the rotating speed S of the induction motor 14, the primary current I and the torque command value T. The torque control unit 25 drives the induction motor 14 by means of the VVVF control corresponding to the torque command value T. The hydraulic pump 9 raises an emitting hydraulic (pump) pressure to provide the pressure oil 2 to the cylinder 1. Thus the cage 6 ascends at a speed corresponding to the speed command value Q.
On the other hand, during descent, the negative-polarity speed command value Q is input into the speed control unit 23 which generates a reversed-polarity torque command value T according to the deviation between the speed command value Q and the rotating speed S, in contrast to the torque command value during ascent.
The induction motor 14 is driven by the VVVF control corresponding to the reversed-polarity torque command value T. The hydraulic pump 9 reduces the emitting hydraulic pressure to drive back the pressure oil in the cylinder 1 to the oil tank 12. Thus the cage 6 descends at a speed according to the speed command value Q.
The speed of the cage 6 can be controlled by variably controlling the amount of oil emitted from the hydraulic pump 9.
In contrast to the operation of the flow control valve, the solenoid-operated valve 7 provided between the hydraulic pump 9 and the cylinder 1 of the hydraulic jack has only a function of opening and closing the pressure oil passage when the elevator is started and stopped. Accordingly, the amount of the opening of the solenoid-operated valve 7 is preset in order that the valve's internal pressure loss is sufficiently minimized.
If the elevator is operated when the solenoid-operated valve 7 is defectively opened by some failure of a part and a pressure loss in the valve 7 is great, the speed control unit 23 generates the larger torque command value T by the pressure loss of the solenoid-operated valve 7 than usual in order to make an actual speed S follow the speed command value Q. As a result, the hydraulic pressure emitted from the hydraulic pump 9 becomes abnormally high during ascent and abnormally low during descent. Since the pump pressure, which decreases during descent, often becomes negative, a failure may occur in the hydraulic pump 9 causing very dangerous condition due to air mixed in the oil pipes. Measures have been taken against the abnormally high pressure of oil of the hydraulic pump 9 which may be generated during ascent. For example, the operation of a relief valve can minimize the effects of failure of certain parts. However, no measures have been taken against abnormally negative pressures of the hydraulic pump 9 which may be generated during descent.
A torque limiter may be provided to the output portion of the speed control unit 23. However, a limiting value of the torque command value T must be set higher than a value within which the ascent can be performed at a predetermined acceleration even with a heavy load. This limiting value cannot absolutely prevent a negative value of the hydraulic pump 9 caused by some failure of the opening of the solenoid-operated valve 7.
In the conventional drive control unit of the hydraulic elevator as described above, no measures have been taken against abnormally negative pressure which may occur in the hydraulic pump 9 due to the failure of the opening of the solenoid-operated valve 7 during descent, which cause a failure to occur in the hydraulic pump and an air to be mixed in the oil pipes.